If enough Ds think this about someone, the legitimate method is for them to vote for someone else in the primary Any other way to prevent this is "party elites putting their fingers on the scale against the legitimate preference of voters/members", and I am assured there is no greater sin
Like when Harris ran instead of Biden?
It didn't happen before he actually WAS incapable in the eyes of enough people also the reason it was Harris is BECAUSE she was chosen by primary voters to back him up if he became incapable, and there was outrage at the idea of skipping her
I mean, that’s what we tell ourselves. It was clearly a mistake to run Biden at his age. There are other examples. Maybe we should try to avoid it happening again?
Maybe you should, but the "you" in question probably needs to be broader than "party officials making rules to freeze out old people/long serving people who remain too popular to beat normally" voters especially need to put more thought in than "do I know this guy?", particularly in primaries
I suggest a hard (high) age cut off.